“Honeycomb monolith substrate” as defined herein includes metal and ceramic flow-through monoliths having a plurality of channels or cells which extend longitudinally along the length of the substrate structure and wherein the channels are open at both ends thereof; and metal and ceramic filters including ceramic wall-flow filters having a plurality of channels or cells which extend longitudinally along the length of the substrate structure and wherein channels at a first end of the substrate that are open are blocked at the opposite end and channels that are open at the opposite end are blocked at the first end, the arrangement being such that every other adjacent cell has an open end (or a blocked end) on the first end of the wall-flow filter and a blocked end (or an open end) on the opposite end thereof so that when an end of the wall-flow filter is viewed it resembles a chess board of open and blocked channels. Fluid communication between the open channels at the first end of the wall-flow filter and the open channels of the opposite end thereof is via the porous wall structure of the wall-flow filter.
The definition “honeycomb monolith substrate” also includes metallic so-called “partial filters” such as that which is disclosed in WO 01/80978 or the substrate disclosed in EP 1057519.
Typically, ceramic materials for manufacturing honeycomb monolith substrates include silicon carbide, aluminium nitride, silicon nitride, aluminium titanate, sintered metal, alumina, cordierite, mullite, pollucite, a thermet such as Al2O3/Fe, Al2O3/Ni or B4C/Fe, or composites comprising segments of any two or more thereof.
The formulation of liquids comprising catalyst components for coating honeycomb monolith substrates are known to those skilled in the art and include: aqueous solutions of platinum group metal compounds, such as platinum, palladium and rhodium compounds, aqueous solutions of alkali metal and alkaline earth metal compounds for depositing compounds for absorbing NOx on the substrates, and other components such as compounds of transition metals e.g. iron, copper, vanadium, cerium and transition metal catalyst promoter compounds; washcoat slurries including particulate catalyst support materials such as alumina, ceria, titania, zirconia, silica-alumina and zeolites, optionally supporting one or more of the above mentioned platinum group metals or transition metals; and washcoat slurries containing combinations of supported metal compounds and aqueous solutions of the above mentioned metal compounds. Such liquids can also include relevant acids, organic compounds thickeners etc. to improve the catalyst activity, chemistry of the formulation to suit the intended purpose of the resulting catalyst, and/or the viscosity and rheology of the liquid.
Apparatus for automatedly coating a honeycomb monolith substrate is known, for example, from our WO 99/47260 and from U.S. Pat. No. 5,422,138. The latter reference discloses an apparatus comprising means for holding a honeycomb monolith substrate substantially vertically and means for introducing a pre-determined volume of a liquid into the substrate via open ends of the channels at a lower end of the substrate.
EP 1325781 discloses a development of the coating technique disclosed in U.S. Pat. No. 5,422,138 that can be used to produce modern “zoned” substrates.